Thermal actuator

ABSTRACT

A thermal actuator including a casing having an opening at one end and a chamber substantially filled with a thermally expandable medium supported within the casing. The chamber includes a neck portion defining an open-ended bore in axial registry with the opening in the casing. A piston assembly including a piston head is received within the bore and has a piston rod projecting from the bore toward the opening in the casing. A self-regulating heat generating element is disposed in thermal communication with the thermally expandable medium and generates heat at a substantially constant temperature sufficient to effect a predetermined expansion of the medium in response to electrical energization of the heating element so as to cause the medium to apply sufficient force against the piston head to effect movement thereof from a first rest position to a second actuated position.

United States Patent Berg 1 Aug. 29, 1972 [54] THERMAL ACTUATOR 72 I t Pete N rt Primary Examiner-Remard A. Gilheany 1 or r g on Mass Assistant Examiner-Dewitt M. Morgan [73] Ass1gnee: Texas Instruments Incorporated, Att0meyHar0ld Levine et al.

Dallas, Tex. 22 Filed: Aug. 3, 1971 [57] ABSTRACT A thermal actuator including a casing having an open- [211 Appl' l68657 ing at one end and a chamber substantially filled with a thermally expandable medium supported within the [52] US; Cl. ..60/23, 73/3683, 236/68 R, casing. The chamber includes a neck portion defining 337/107, 337/393 an open-ended bore in axial registry with the opening 51- Int. Cl. ..F03g 7/06,G05d 15/01 in the g- A piston s y including a piston 58 Field 0f Search ..60/23; 73/3683; 236/68 R, received within the bore and has a piston rod 236/100, 101;337/107, 114, 115,118,120, p s from the toward the qp in e 121 393 casing. A self-regulatmg heat generatlng element 1s disposed in thermal communication with the thennally References Cited expandable medium and generates heat at a substantially' constant temperature sufficient to efiect a UNITED STATES PATENTS predetermined expansion of the medium in response to electrical energization of the heating element so as to cause the medium to apply sufficient force against 343l726 3/1969 I ase es e 23 the piston head to eflect movement thereof from a 3404530 /1968 2 first rest position to a second actuated position. 3,330,480 7/1967 Drapeau et a1. ..236/ 21 Claims, 5 Drawing Figures minimum I912 3.6861357 sum 1 OF 4 I $0 I Q A26. 5 Q

i' E I 2 3 64,991 g B) Pele! 6. Berg P'A'IENTEmum m2 3.686357 sum 2 or 4 INVENTOR 5): Refer 6. Bery 9mm 5 Wa THERMAL ACTUATOR This invention relates generally to thermal actuators and more particularly is directed to an improved thermal actuator for translating electrical energization into mechanical movement.

A wide variety of actuators have been developed which are adapted to be energized in response to the application of heat to a thermally expansible material, such as various types of wax. Such actuators may typically be utilized in numerous applications and may be utilized for sensing heat such as in automotive applications in which a response to changes in ambient or motor temperature is desired or for operating in response to externally applied heating. For example, certain actuators have been developed in which a resistance heating element is coupled to the thermally expansible material so as to apply heat thereto in order to effect operation of the actuator. The latter variety of actuator has found a certain degree of application in various types of residential and industrial heating systems in which the operation of a temperature sensitive thermostat controls the application of electrical power to the resistance heating element so as to operate a valve, a switch, or the like. However, such actuators have suffered from a number of problems. Initially, the heating element is subject to continued energization until a limit switch senses full valve travel and effects disruption of the electrical power being supplied to the resistance heating element. Such a system, of course, may present a substantial fire hazard in the event of eventual stroke loss which prevents deenergization of the heater since the limit switch remains actuated. In order to overcome such deficiencies heat sensing thermostats may be necessary for limiting the electrical energization and to prevent overheating which substantially adds to the expense and complexity of the system. In addition, such devices have in certain instances failed to exhibit a high degree of durability in view of the high temperature operation required and continuous on-off cycling, as well as the difficulty of providing adequate sealing arrangements for the thermally expansible material. Furthermore, such devices have suffered a certain degree of inaccuracy due to inadequate calibration procedures.

Accordingly, it is an object of the present invention to provide an improved thermal actuator in which substantially constant electrical energization may be employed, while substantially avoiding overheating.

It is another object of the present invention to provide an improved thermal actuator in which the necessity for external devices for limiting electrical energization is substantially eliminated.

It is a further object of the present invention to provide an improved thermal actuator in which continued electrical energization of a heating element may be effected in order to maintain a substantially constant temperature.

It is a still further object of the present invention to provide an improved thermal actuator which is extremely durable in operation and economical to fabricate.

Various additional objects and advantages of the present invention will be readily apparent from the following detailed description and accompanying drawings wherein:

FIG. 1 is a vertical sectional view of one embodiment of a thermal actuator in accordance with the present invention;

FIG. 2 is a sectional view taken along lines 2-2 of FIG. 1;

FIG. 3 is a vertical sectional view of another embodiment of a thermal actuator in accordance with the present invention;

FIG. 4 is a view similar to that illustrated in FIG. 3 but showing the device in an actuated condition; and

FIG. 5 is a perspective view of a complete unit in accordance with the present invention.

Referring generally to the drawings and initially in particular to FIG. 1 a first embodiment of a thermal actuator is illustrated and designated generally by reference numeral 10. The thermal actuator is illustrated in FIG. 1 in a generally vertical orientation and includes an outer casing 12 having an opening 14 at one end and being substantially closed at its opposite end. A generally cup-shaped chamber or container 16 is supported within the casing 12 having its base arranged generally adjacent the closed end of the casing 12 and including an oppositely disposed elongated neck portion 18 which defines a bore in axial registry with the opening 14 but spaced therefrom. The chamber 16 is substantially completely filled with a preselected thermally expandable medium 20 having thermal expansion properties such that its volume expands in response to the elevation of its temperature to a preselected level. A self-regulating heat generating element 22 is disposed in thermal communication with the medium through the wall of the chamber 16, and is adapted to generate heat at a substantially constant temperature level sufficient to heat the medium to the preselected level required for the requisite expansion of the medium 20 in response to electrical energization. The self-regulating heat generating element 22 preferably has a positive temperature coefficient of re sistance and a sharply defined transition temperature above which its resistance increases rapidly so as to substantially maintain the medium heated to the preselected temperature level in response to continuous electrical energization. In addition, a movable member 24 is provided having a force receiving surface 26 and a force applying surface 28. The movable member 24 preferably comprises a piston assembly having a piston head 30 slidingly received within the bore 18 and is adapted to be moved from a first rest or retracted position to a second actuated position in response to expansion of the medium. The piston assembly 24 also includes a piston rod 32 which terminates in the force applying surface 28 and is slidingly received within the opening 14 of the casing so as to move outwardly therefrom in response to actuation of the piston assembly. In addition, sealing means 34 are disposed adjacent the force receiving surface 26 of the piston 30 within the bore 18 for sealingly mounting the piston assembly with respect to the medium within the chamber. In order to maintain the piston assembly 24 in its retracted or rest position, means 36 are provided within the casing 12 for biasing or maintaining the piston assembly in its first retracted position, in the absence of energization.

More particularly, the casing 12 may be fabricated of a single unitary member, but in the illustrated embodiment comprises upper and lower sections 12a, 12b respectively in order to facilitate fabrication thereof. In this regard the lower portion 12b of the casing may be fabricated of a relatively durable inexpensive nonconductive material such as phenolic plastic having a circumferential outwardly projecting shoulder portion 120 to facilitate attachment thereto of the upper portion 12a of the casing. The upper portion 12a of the casing may in certain instances also comprise a suitable nonconductive material such as phenolic plastic, but in the illustrated embodiment is fabricated of a relatively inexpensive light weight durable metallic material such as aluminum with the lower portion thereof being crimped about the shoulder portion 12c of the lower portion 12b of the casing, while its upper end defines the opening 14 therein for slidingly receiving the piston head 32.

The chamber or container 16 is of a generally cupshaped configuration including a base which in the illustrated embodiment is rockably supported within the lower section 12b of the casing, as will be further explained in detail hereinafter in order to compensate for piston misalignment within the bore 18. More particularly, the chamber 16 also includes the relatively elongated neck portion, which defines the bore 18 projecting upwardly through at least a portion of the upper portion 12a of the casing and in general registration with opening 14 for slidably receiving the piston head 30 therein. In addition, the bore 18 includes an aperture 38 in the wall thereof at a location adjacent its upper end which defines a vent passageway through which excess quantities of the medium 20 may pass during calibration of the device, as will be subsequently explained in detail. The chamber 16 is preferably fabricated of a relatively durable, non-corrosive material having good thermal conduction properties, such as brass, copper, etc.

The thermally expandable medium 20 substantially completely occupies the interior of the chamber 16 except for the bore 18 which receives the piston head 30. In this connection the thermally expandable medium is selected to have a temperature coefficient of expansion which is suited to a particular application and may comprise a suitable distilled wax having preselected thermal expansion characteristics such that it transmits force to effect movement of the piston assembly in response to the elevation of its temperature and associated expansion. In this regard various types of wax, polyethylene, etc., may be employed which undergo a relatively abrupt phase charge from a solid to a liquid state and associated expansion in response to being heated to a predetermined temperature. Alternatively, various types of oils may be utilized which exhibit a substantially linear rate of expansion in response to heating. For example, the medium 20 may have characteristics such that it remains in a substantially solid contracted state up to approximately 60 C. but changes to a liquid state and expands by approximately IOpercent of its volume in response to a temperature rise to 70 C. so as to apply sufficient force against the force receiving surface 26 of the piston head 30 to cause the piston head to move to a position approximately midway along the length of the bore 18 at which point the piston rod 32 is in a fully actuated position and applies a predetermined force against or effects corresponding movement of an external member engaging its exposed force applying surface 28.

In accordance with an extremely significant feature of the present invention heat is applied to the medium 20 so as to maintain the medium at a substantially constant elevated temperature and state of expansion sufficient to maintain the piston assembly in its actuated condition, while obviating the possibility of heating the medium to a point beyond this temperature level without the need for any additional limit switch devices, or the like to effect disruption of the heating. in this connection the heating element 22 is disposed in a position thermally coupled to the external wall of chamber 16, preferably mounted in juxtaposition therewith, and is adapted to generate heat at a substantially constant temperature in response to continuous electrical energization thereof. More particularly, the heating element 22 comprises a self-regulating positive temperature coefiicient of resistance thermistor having a steeply-sloped positive temperature coefficient of resistance at temperatures above an anomaly temperature, whereby the application of electrical energization to the heating element 22 causes it to begin to self heat and reach a predetermined temperature level which remains substantially constant during continuous electrical energization thereof as a result of the substantial increase in resistance of the element at this temperature level. Aparticularly advantageous heating element comprises a ceramic wafer comprising semiconducting barium titanate, such as Ba La Tiol In addition, in the illustrated embodiment the heating element 22 is mounted externally of the medium 20 so as to avoid any possibility of contamination thereof, but is mounted within the interior of the casing 12 so as to provide protection therefor. Electrical energization of the heating element 22 is provided by a pair of electrical conductors 40, 42 which are respectively coupled to opposed contact surfaces of the element 22 to effect the passage of electrical current thereto. More particularly, the conductor 40 is coupled to an exposed contact surface of the element 22 as shown and extends through an eyelet 44 in the base of the casing 12 and is electrically connected to an exposed male terminal pin 46. In addition, since the opposite contact surface of the element 22 is in contact with the base of chamber 16 to maximize thermal coupling to the medium 20, electrical contact thereto if effected by maintaining the conductor 42 in contact with the wall of the chamber 16, as shown, while the opposite end of the conductor 42 extends through a similar eyelet 48 in the casing 12 and is electrically connected to another male terminal pin 50. The terminal pins 46, 50 are mutually electrically insulated from each other and in order to provide further electrical insulation therebetween a portion 52 of the nonconductive casing material projects downwardly from the casing intermediate the eyelets 44, 48. The male terminal pins 46, 50 are adapted to be connected to a suitable source of electrical power which may be an ac. or d.c. source to effect energization of the heating element 22.

If desired the container 16 may have a generally arcuate circumferential edge defined by the junction between its base and its side wall. Accordingly, rockable support for the container in order to compensate for piston misalignment within its bore 18 may be conveniently provided by arranging a plurality of generally upright support standards 54 extending from the base of the lower casing portion 12b, as particularly illustrated in FIG. 2, for supportingly receiving the arcuate circumferential edge of the chamber 16.

As previously mentioned, the piston assembly 24 includes the piston head 30 which is slidingly received within the bore 18 and adapted to undergo reciprocating motion therein and the piston rod 32 which projects from the opposite end of the piston assembly and is slidingly received within the opening 14 for undergoing reciprocating motion therein between a first position of rest and a second actuated position in which it may project outwardly from the casing a predetermined distance in order to apply a force to an external member, to effect movement of an external member, etc. In the illustrated embodiment in order to minimize the length of the piston assembly, while permitting the desired reciprocating movement thereof as well as to enable the assembly to be conveniently biased in the first position of rest, the assembly includes an integral circumferential shoulder member 60 which in the illustrated embodiment depends downwardly from the piston rod 32 and surrounds at least a portion of the bore 18 such that the bore extends in the space intermediate the piston head 30 and the depending shoulder member 60. In addition, the shoulder member 60 includes an integral flange 62 which projects outwardly therefrom and bears against the interior of the casing portion 124 to aid in maintaining the path of movement of the piston. The flange also serves an additional important function in that the means 36 for biasing the piston assembly in its retracted or rest position and effecting return thereof to this retracted position subsequent to actuation preferably comprises a spring, as shown, which is disposed intermediate the depending shoulder portion and the interior of the casing bearing against the upper portion of the flange 62 and the upper internal wall of the casing portion 12a adjacent the opening 14. The piston assembly 24 is arranged such that the force receiving surface 26 of the piston head is in a position adjacent the junction defined between the chamber 16 and the bore 18, when the piston assembly is maintained in its rest position by the spring 36. In addition, as shown, the force receiving surface 26 is preferably in engagement with the medium 20 so that expansion of the medium effects corresponding movement of the piston assembly.

In order to facilitate provision of the sealing means 34 for precluding leakage of the medium 20 from the chamber 16 the piston head 30 is preferably arranged to include an outwardly projecting member 64 extending toward the interior of chamber 16 and defining the force receiving surface 26 at the end thereof. The projecting member 64 is of a somewhat lesser diameter than the remainder of the piston head 30 and of the bore 18 so as to define a space 66 intermediate the projecting portion 64 and the wall of the bore 18 for acco'mmodating the sealing means 34, which preferably comprises an O-ring. The O-ring 34 is disposed within the space 66 in surrounding relationship with at least a part of the projecting member 64. The O-ring is preferably fabricated of a relatively flexible durable rubber material capable of continuous high temperature operation and in one example comprises a fluoroelastomer material such as the material designated by the trade name Viton sold by El. Dupont de Nemours and Company. The O-ring 34 is maintained in position against the piston head as a result of the pressure exerted thereagainst by the medium 20 within the chamber which, as previously explained, substantially fills the chamber and serves to maintain the O-ring 34 in the requisite position for defining a seal. Similarly, the spring 36 provides an opposite force to further aid in maintaining the seal. As the piston moves between its rest and actuated positions the O-ring 34 is similarly maintained in sealing relationship with the piston head due to the pressure of the medium and hence a reciprocating elastomeric seal is defined.

To briefly review operation of the thermal actuator illustrated in FIG. 1, upon application of electrical power to the terminal members 46, the heating element 22 is caused to begin to self-heat and reaches and maintains a substantially constant predetermined temperature level. The heat generated effects corresponding heating of the medium 20 and effects expansion, thereby forcing the piston head 30 to move upwardly within the bore a predetermined distance toward its actuated position. Since the temperature of the heating element remains substantially constant the medium 20 expands in volume by only a predetermined amount, and this state of expansion is maintained as long as the heating element is electrically energized. Upon disruption of the electrical energization of the heating element the medium 20 begins to cool and correspondingly retract to its original volume and the piston head 30 similarly returns to its initial rest or retracted position due to the downward pressure exerted thereon by the spring 36.

In accordance with another important feature of the present invention the vent passage 38 is provided in order to facilitate calibration of the device, which may be accomplished quite conveniently during fabrication. More particularly, during fabrication a surplus quantity of the medium 20 is preferably disposed within the chamber 16 and heating is applied at a temperature substantially in excess of that to which the medium is to be exposed during operation. Accordingly, the force receiving surface 26 of the piston head and the associated seal 34 are caused to be moved to a position along the bore 18 beyond the vent passageway 38 as a result of the excess quantity of the medium and its abnormal expansion due to the excessive temperature, thereby exposing the vent passageway 38 to the heated medium. As a result the medium flows outwardly through the vent passageway 38 so that surplus quantities are expelled from the container 16. After the level of medium within the bore 18 has reached the position defined by passageway 38 the heat is removed and the medium contracts, permitting the piston head to similarly return to its rest position. Since a portion of the medium has been expelled and further since the device will not be exposed to such elevated temperatures during subsequent usage, the medium cannot subsequently expand sufficiently to force the piston head 30 and the seal 34 to a position at which the vent passageway 38 is exposed to the medium 20. Accordingly, the device is thus calibrated for subsequent usage since the piston head cannot again reach the level in the bore defined by the vent passageway 38.

Such calibration techniques eliminate the need for the use of mechanical restraints, deformation of the casing, etc., for preventing piston travel beyond a certain point in order to effect calibration.

Referring now to FIG. 3 another embodiment of the present invention is illustrated which is substantially similar to that shown in FIG. 1 and corresponding elements are accordingly indicated by corresponding reference numerals. In this embodiment in order to accelerate heating of the medium a heat radiating element 70 is disposed within the chamber 16 abutting the base thereof in thermal communication with the heating element 22 and extending through a portion of the medium so as to distribute the heat being generated by the heating element thereby effecting more rapid expansion of the medium 20 in response to energization of the heating element. The heat radiating element 70 may comprise a plurality of vanes or fins of a thermally conductive material or as in the illustrated embodiment may comprise an open-ended cup-shaped container fabricated of a thermally conductive material, such as brass or copper. In addition, in this embodiment a modified piston head 72 is provided which is somewhat different from the piston head 30 of the preceding embodiment. More particularly, in order to obviate the necessity for rockably mounting the chamber 16 to compensate for piston misalignment within the bore 18, in this embodiment the chamber 16 is rigidly mounted within the casing by the provision of suitable ribs 74, or the like. The piston head 72 which is received within the bore 18 is arranged to have a substantially lesser diameter than the bore 18 thereby permitting free and unencumbered movement of the piston head within the bore even in the event of gross misalignment therebetween. However, in order to permit the medium to effect application of the requisite force against the force receiving surface 76 of the piston head 72, while providing a seal between the medium 20 and the external environment a floating head piston 78 is disposed within the bore 18 intermediate the medium 16 and the force receiving surface 76 of the piston head 72. As shown, the floating head piston 78 preferably comprises a generally T-shaped member including a head 80 which bears against the force receiving surface 76 of the piston head 72, a body member 82 which is of a smaller diameter than the bore 18 so as to define a space between the bore and the body to accommodate an O-ring seal 84, and a base 86 which is in opposition with the head 80 and is adapted to be engaged by the medium 20 within container 16 upon expansion for effecting movement of the piston head 72. In this connection the floating head piston 78 is preferably fabricated of a relatively light-weight material, such as aluminum, and is maintained in its requisite position within the bore as a result of a friction-fit provided by the O-ring seal 84, which again is preferably fabricated of a fluoro-elastomer material, such as Viton. The O-ring 84 is disposed in surrounding relationship with a portion of the body 82 of the floating head piston, as shown. Since The O-ring is of a flexible material it deforms to compensate for any non-concentricity of the floating head piston 78 with respect to the bore 18 and functions to maintain the floating head piston 78 centered within the bore. In addition to the support thus provided by the O-ring seal 84, the head abuts and bears against the surface 76 of the piston head 72 so that the floating head piston is precluded from pivotal movement within the bore even in the event of relatively gross misalignment. The O-ring seal 84 functions to maintain the requisite seal both when the medium is in a contracted state within the chamber, as well as when the medium is in an expanded state in which it effects movement of the piston assembly and occupies a portion of the bore 18, as well as engaging the O-ring 84.

Operation of the embodiment illustrated in FIG. 3 is essentially the same as that described in connection with the FIG. 1 embodiment except that expansion of the medium 20 in response to the application of heat thereto accompanying energization of the heating element 22 causes the medium to expand and force the floating head piston upwardly in the bore 18 thereby effecting movement of the piston head 72 'within the bore. Accordingly, the floating head piston functions as an additional force-transmitting element. To further illustrate'operation, the device of FIG. 3 is illustrated in FIG. 4 with the piston assembly in an actuated position corresponding to expansion of the medium 16 within the container due to heating thereof. Upon disruption of the heating the spring 36 forces the piston assembly to return to its rest position and similarly causes the piston head 72 to force the floating head piston 78 to return to its initial rest position wherein it is supported by the medium 20 within the chamber 16. Similarly, calibration of this embodiment may be effected as explained in connection with the FIG. 1 embodiment described hereinabove.

Referring now to FIG. 5 a perspective view of a completed device such as shown and described in FIGS. 1-4 is illustrated showing the casing 12 including the upper and lower portions 12a, 12b thereof as well as the terminal members 46, 50, and the piston rod 32 and its associated force applying surface 28 projecting from the casing.

Thus, a unique thermal actuator has been shown and described in which electrical energization is translated into physical movement, which is extremely economical to fabricate and utilize without the need for additional protective equipment as a result of the self-regulating heating function. Various changes and modifications in the above-described embodiments will be readily apparent to those skilled in the art and any of such changes or modifications are deemed to be within the spirit and scope of the present invention as set forth in the appended claims.

Iclaim:

1. A thermal actuator comprising a casing having an opening at one end thereof,

a movable member having a force applying surface and a force receiving surface supported within said casing and adapted to be moved from a first retracted position to a second actuated position,

means coupled to said movable member for maintaining said member in said first retracted position and effecting return thereto subsequent to actuation,

a container supported within said casing and spaced from said one end thereof, said container having an opening at one end adapted to accommodate a portion of said movable member including the force receiving surface thereof,

means disposed within said opening in said container for sealingly mounting said portion of said movable member with respect to the interior of said container,

a thermally expansible, force-transmitting medium disposed within the interior of said container, said medium being adapted to apply a preselected force to the force receiving surface of said movable member so as to effect movement of said movable member toward its actuated position in response to a predetermined elevated temperature of said medium and associated expansion thereof, and heating means mounted in thermal communication with said medium, said heating means including a self-regulating heating element adapted to generate heat in response to electrical energization thereof and having a steeply-sloped positive temperature coefficient of resistance at temperatures above an anomaly temperature so as to generate heat at a substantially constant temperature in order to maintain said predetermined elevated temperature of said medium responsive to continuous electrical energization.

2. A thermal actuator in accordance with claim 1 wherein said movable member comprises a piston assembly adapted for reciprocal movement, said force receiving surface being disposed in communication with the interior of said container in force-receiving relationship with said medium.

3. A thermal actuator in accordance with claim 2 wherein said piston assembly includes a piston head adapted to be slidingly received within said opening at said one end of said container, said piston head defining said force receiving surface at an exposed end thereof and including a piston rod extending from said piston head defining an opposite end of said piston assembly and defining said force applying surface at its exposed end, said piston rod being adapted to be slidingly received within the opening at said one end of said casing.

4. A thermal actuator in accordance with claim 3 wherein said piston head and piston rod are generally cylindrical and an integral circumferential shoulder member extends from said piston rod including a depending portion surrounding a portion of said container including the end thereof which receives said piston head.

5. A thermal actuator in accordance with claim 4 wherein said piston rod includes an integral flange projecting outwardly from said depending portion, said flange bearing against the interior of said casing to aid in maintaining the path of movement of said piston.

6. A thermal actuator in accordance with claim 5 wherein said means for maintaining said piston in said first retracted position and efi'ecting return thereof to said first retracted position subsequent to actuation comprises a spring extending between said flange and an opposing inner surface of said'casing, said spring being disposed intermediate said depending shoulder portion and said casing.

7. A thermal actuator in accordance with claim 3 wherein said container comprises a generally cupshaped member having an open-ended generally elongated, thin neck defining a bore terminating in said opening at said one end of said container, said bore being in registration with said opening in said casing for 10 receiving said piston head, and a reciprocating elastomeric seal disposed within said bore adjacent said force receiving surface of said piston head to substantially prevent undesired leakage of said medium from said container.

8. A thermal actuator in accordance with claim 7 wherein said reciprocating elastomeric seal includes a flexible O-ring.

9. A thermal actuator in accordance with claim 8 wherein said piston head is slidingly mounted within said bore and includes a projecting member extending outwardly therefrom into said bore toward the interior of said container, said projecting member being spaced from the wall of said bore, and said O-ring is disposed intermediate said projecting member and the wall of said bore in surrounding relationship with a portion of said projecting member.

10. A thermal actuator in accordance with claim 7 wherein said piston head is of a substantially smaller diameter than said bore, a force-transmitting floating head piston is disposed within said bore intermediate the force receiving surface of said piston head and said medium, and said reciprocating elastomeric seal is disposed within said bore adjacent said floating head piston for supportingly maintaining said floating head piston centered within said bore and preventing said medium from passing said floating head piston.

11. A thermal actuator in accordance with claim 10 wherein said floating head piston includes a portion having a diameter which is less than that of said bore so as to define a space between said portion and said bore and said reciprocating elastomeric seal comprises an O- ring disposed within said space, thereby providing a seal between said medium and said piston head.

12. A thermal actuator in accordance with claim 11 wherein said self-regulating heating element includes a ceramic wafer comprising semiconducting barium titanate disposed adjacent to the external wall of said container and in heat transfer relationship with said medium.

13. A thermal actuator in accordance with claim 12 wherein said ceramic wafer comprises Ba l.a,, Ti0

14. A thermal actuator in accordance with claim 11 wherein said self-regulating heating element comprises a positive temperature coefficient thermistor and is adapted to maintain a constant elevated temperature in response to continued electrical energization thereof.

15. A thermal actuator in accordance with claim 12 wherein a heat radiating element of a thermally conductive material is disposed in thermal communication with said self-regulating heating element and with said medium for transmitting heat through said medium at an accelerated rate.

16. A thermal actuator in accordance with claim 7 wherein said container includes a base in opposition with said bore, said base being peripherally defined by a circumferential arcuate edge, said base being spaced from said casing, and a plurality of support members secured within said casing rockably support said container at said arcuate edge.

17. A thermal actuator in accordance with claim 7 wherein a calibration means including a vent passage is defined in the wall of said bore longitudinally spaced from the path of travel of said elastomeric seal and separated from said medium by said seal, said vent passage being adapted to pass surplus quantities of said medium therethrough when said medium is subjected to additional heating substantially beyond the temperature capacity of said heating element effecting expansion thereof and consequent piston head movement beyond said actuated position whereby said vent passage drains excess quantities of said medium so that subsequent heating applied by said heating element cannot cause sufficient expansion of the remaining medium to effect movement of said piston head beyond said actuated position.

18. A thermal actuator comprising a casing having an opening at one end and being substantially closed at an opposite end,

a generally cup-shaped chamber supported within said casing, said chamber having a base arranged adjacent the closed end of said casing and having an oppositely disposed elongated neck portion defining an open-ended'bore in axial registry with said opening in said casing and spaced therefrom,

a thermally expandable medium carried within and substantially filling the interior of said chamber, said medium being adapted to expand in volume in response to an elevation of the temperature thereof to a preselected level,

a self-regulating heat generating element disposed in thermal communication with said medium and spaced therefrom by the wall of said chamber, said heating element being adapted to generate heat at a substantially constant temperature substantially equal to said preselected level in response to electrical energization thereof and having a positive temperature coefficient of resistance so as to substantially maintain said preselected level responsive to continuous electrical energization thereof,

' a piston assembly including a piston head received within said bore and having an external face at one end oriented toward the interior of said chamber, a floating head piston slidingly mounted within said bore intermediate the external face of said piston head and the interior of said chamber for transmitting force applied thereto by expansion of said medium to effect movement of said piston head in said bore, and a piston rod projecting outwardly from an opposite end of said piston head toward said opening in said casing and adapted to undergo movement between first and second positions in response to expansion of said medium, and

means coupled to said piston assembly for maintaining said piston rod in said first position prior to expansion of said medium and for effecting return of said piston rod to said first position upon contraction of said medium.

19. A thermal actuator in accordance with claim 18 wherein said medium substantially completely fills the interior of said chamber, said floating head piston is supported within said bore and a movable elastomeric seal is disposed within said bore intermediate a portion of said floating head piston and said medium to maintain said floating head piston concentric with respect to said bore and preclude leakage of said medium from said container during movement of said piston asi t li k thermal actuator in accordance with claim 18 wherein said self-regulating heating element is mounted exteriorly of said chamber and within said casing.

21. A thermal actuator in accordance with claim 20 wherein said self-regulating heating element comprises a positive temperature coefficient of resistance thermistor formed of semiconducting barium titanate. 

1. A thermal actuatOr comprising a casing having an opening at one end thereof, a movable member having a force applying surface and a force receiving surface supported within said casing and adapted to be moved from a first retracted position to a second actuated position, means coupled to said movable member for maintaining said member in said first retracted position and effecting return thereto subsequent to actuation, a container supported within said casing and spaced from said one end thereof, said container having an opening at one end adapted to accommodate a portion of said movable member including the force receiving surface thereof, means disposed within said opening in said container for sealingly mounting said portion of said movable member with respect to the interior of said container, a thermally expansible, force-transmitting medium disposed within the interior of said container, said medium being adapted to apply a preselected force to the force receiving surface of said movable member so as to effect movement of said movable member toward its actuated position in response to a predetermined elevated temperature of said medium and associated expansion thereof, and heating means mounted in thermal communication with said medium, said heating means including a self-regulating heating element adapted to generate heat in response to electrical energization thereof and having a steeply-sloped positive temperature coefficient of resistance at temperatures above an anomaly temperature so as to generate heat at a substantially constant temperature in order to maintain said predetermined elevated temperature of said medium responsive to continuous electrical energization.
 2. A thermal actuator in accordance with claim 1 wherein said movable member comprises a piston assembly adapted for reciprocal movement, said force receiving surface being disposed in communication with the interior of said container in force-receiving relationship with said medium.
 3. A thermal actuator in accordance with claim 2 wherein said piston assembly includes a piston head adapted to be slidingly received within said opening at said one end of said container, said piston head defining said force receiving surface at an exposed end thereof and including a piston rod extending from said piston head defining an opposite end of said piston assembly and defining said force applying surface at its exposed end, said piston rod being adapted to be slidingly received within the opening at said one end of said casing.
 4. A thermal actuator in accordance with claim 3 wherein said piston head and piston rod are generally cylindrical and an integral circumferential shoulder member extends from said piston rod including a depending portion surrounding a portion of said container including the end thereof which receives said piston head.
 5. A thermal actuator in accordance with claim 4 wherein said piston rod includes an integral flange projecting outwardly from said depending portion, said flange bearing against the interior of said casing to aid in maintaining the path of movement of said piston.
 6. A thermal actuator in accordance with claim 5 wherein said means for maintaining said piston in said first retracted position and effecting return thereof to said first retracted position subsequent to actuation comprises a spring extending between said flange and an opposing inner surface of said casing, said spring being disposed intermediate said depending shoulder portion and said casing.
 7. A thermal actuator in accordance with claim 3 wherein said container comprises a generally cup-shaped member having an open-ended generally elongated, thin neck defining a bore terminating in said opening at said one end of said container, said bore being in registration with said opening in said casing for receiving said piston head, and a reciprocating elastomeric seal disposed within said bore adjacent said force receiving surface of said piston head to substantially prevent undesired leakAge of said medium from said container.
 8. A thermal actuator in accordance with claim 7 wherein said reciprocating elastomeric seal includes a flexible O-ring.
 9. A thermal actuator in accordance with claim 8 wherein said piston head is slidingly mounted within said bore and includes a projecting member extending outwardly therefrom into said bore toward the interior of said container, said projecting member being spaced from the wall of said bore, and said O-ring is disposed intermediate said projecting member and the wall of said bore in surrounding relationship with a portion of said projecting member.
 10. A thermal actuator in accordance with claim 7 wherein said piston head is of a substantially smaller diameter than said bore, a force-transmitting floating head piston is disposed within said bore intermediate the force receiving surface of said piston head and said medium, and said reciprocating elastomeric seal is disposed within said bore adjacent said floating head piston for supportingly maintaining said floating head piston centered within said bore and preventing said medium from passing said floating head piston.
 11. A thermal actuator in accordance with claim 10 wherein said floating head piston includes a portion having a diameter which is less than that of said bore so as to define a space between said portion and said bore and said reciprocating elastomeric seal comprises an O-ring disposed within said space, thereby providing a seal between said medium and said piston head.
 12. A thermal actuator in accordance with claim 11 wherein said self-regulating heating element includes a ceramic wafer comprising semiconducting barium titanate disposed adjacent to the external wall of said container and in heat transfer relationship with said medium.
 13. A thermal actuator in accordance with claim 12 wherein said ceramic wafer comprises Ba0.997La0.003Tio3.
 14. A thermal actuator in accordance with claim 11 wherein said self-regulating heating element comprises a positive temperature coefficient thermistor and is adapted to maintain a constant elevated temperature in response to continued electrical energization thereof.
 15. A thermal actuator in accordance with claim 12 wherein a heat radiating element of a thermally conductive material is disposed in thermal communication with said self-regulating heating element and with said medium for transmitting heat through said medium at an accelerated rate.
 16. A thermal actuator in accordance with claim 7 wherein said container includes a base in opposition with said bore, said base being peripherally defined by a circumferential arcuate edge, said base being spaced from said casing, and a plurality of support members secured within said casing rockably support said container at said arcuate edge.
 17. A thermal actuator in accordance with claim 7 wherein a calibration means including a vent passage is defined in the wall of said bore longitudinally spaced from the path of travel of said elastomeric seal and separated from said medium by said seal, said vent passage being adapted to pass surplus quantities of said medium therethrough when said medium is subjected to additional heating substantially beyond the temperature capacity of said heating element effecting expansion thereof and consequent piston head movement beyond said actuated position whereby said vent passage drains excess quantities of said medium so that subsequent heating applied by said heating element cannot cause sufficient expansion of the remaining medium to effect movement of said piston head beyond said actuated position.
 18. A thermal actuator comprising a casing having an opening at one end and being substantially closed at an opposite end, a generally cup-shaped chamber supported within said casing, said chamber having a base arranged adjacent the closed end of said casing and having an oppositely disposed elongated neck portion defining an open-ended bore in axial reGistry with said opening in said casing and spaced therefrom, a thermally expandable medium carried within and substantially filling the interior of said chamber, said medium being adapted to expand in volume in response to an elevation of the temperature thereof to a preselected level, a self-regulating heat generating element disposed in thermal communication with said medium and spaced therefrom by the wall of said chamber, said heating element being adapted to generate heat at a substantially constant temperature substantially equal to said preselected level in response to electrical energization thereof and having a positive temperature coefficient of resistance so as to substantially maintain said preselected level responsive to continuous electrical energization thereof, a piston assembly including a piston head received within said bore and having an external face at one end oriented toward the interior of said chamber, a floating head piston slidingly mounted within said bore intermediate the external face of said piston head and the interior of said chamber for transmitting force applied thereto by expansion of said medium to effect movement of said piston head in said bore, and a piston rod projecting outwardly from an opposite end of said piston head toward said opening in said casing and adapted to undergo movement between first and second positions in response to expansion of said medium, and means coupled to said piston assembly for maintaining said piston rod in said first position prior to expansion of said medium and for effecting return of said piston rod to said first position upon contraction of said medium.
 19. A thermal actuator in accordance with claim 18 wherein said medium substantially completely fills the interior of said chamber, said floating head piston is supported within said bore and a movable elastomeric seal is disposed within said bore intermediate a portion of said floating head piston and said medium to maintain said floating head piston concentric with respect to said bore and preclude leakage of said medium from said container during movement of said piston assembly.
 20. A thermal actuator in accordance with claim 18 wherein said self-regulating heating element is mounted exteriorly of said chamber and within said casing.
 21. A thermal actuator in accordance with claim 20 wherein said self-regulating heating element comprises a positive temperature coefficient of resistance thermistor formed of semiconducting barium titanate. 